This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We are developing software for interactively analyzing 3-dimensional density maps obtained by electron microscopy (EM). The analysis aims to determine structures and inner workings of molecular machines such as viruses, ribosomes, microtubules and motors, muscle filaments, and dozens of others systems that are targets of current research. It is believed that most proteins in living organisms are parts of large molecular assemblies. Advances in experimental methods in the past several years have greatly accelerated research on these systems. The primary database of EM density maps (http://www.ebi.ac.uk/msd-srv/emsearch/index.html) was founded in 2002 and now (2010) has 773 entries. Software for deducing structures from these maps is being actively developed by many labs. Our software displays contour surfaces of density maps. Maps from electron microscopy have resolutions in the 5 - 100 Angstrom range which is too coarse to see atomic detail. Analysis involves fitting known atomic structures from crystallography into the maps, identifying structures in maps corresponding to unidentified proteins, and comparing maps of the same system under different experimental conditions to deduce conformational changes or binding of ligands or specific macromolecules. Our software tools, which are part of the UCSF Chimera molecular modeling package, support fitting, carving out density regions, coloring maps, and building coarse models in maps. In the past we have added the ability to slice maps along an arbitrarily oriented and movable plane displaying density values on the cut surface.